Hybrid window layer for photovoltaic cells

ABSTRACT

A novel photovoltaic solar cell and method of making the same are disclosed. The solar cell includes: at least one absorber layer which could either be a lightly doped layer or an undoped layer, and at least a doped window-layers which comprise at least two sub-window-layers. The first sub-window-layer, which is next to the absorber-layer, is deposited to form desirable junction with the absorber-layer. The second sub-window-layer, which is next to the first sub-window-layer, but not in direct contact with the absorber-layer, is deposited in order to have transmission higher than the first-sub-window-layer.

CROSS-REFERENCE TO RELATED APPLICATIONS AND STATEMENT REGARDINGSPONSORED RESEARCH

The present invention is a divisional application of the patentapplication Ser. No. 10/696,545 filed Oct. 29, 2003.

This invention was made with Government support under National RenewableEnergy Laboratory (NREL) contract No. NDJ-1-30630-08 awarded by theDepartment of Energy. The government has certain rights in thisinvention.

TECHNICAL FIELD

The present invention relates to a novel doped window layer andphotovoltaic solar cells containing the same.

BACKGROUND OF THE INVENTION

Solar cells rely on the semiconductor junction to convert sunlight intoelectricity. The junction consists at least of two layers of oppositetypes one layer being an n-layer with an extra concentration ofnegatively charged electrons and the other layer being a p-layer with anextra concentration of positively charged holes. There is at least awindow layer, which is usually heavily doped and an absorber layer,which is either a lightly doped or undoped semiconductor. In solarcells, only photons that are near or above the semiconductor bandgap ofthe absorber layer can be absorbed and utilized. In the solar radiation,there is a limited amount of flux of photons with energy above such avalue. Unfortunately, all photons will have to pass through the dopedwindow layer before the photons reach the absorber layer. Those photonsabsorbed by the window layer will not be able to be converted intouseful electricity and are wasted. One way to reduce such an absorptionis to make the doped window layer with a wider bandgap and to make thedoped window layer very thin. However, a minimum thickness is requiredfor the doped window layer in order to maintain build-in potential. Whenthe bandgap of the window layer is increased beyond the absorber layer,there is a mismatch in the band edge at the junction. Such a mismatch atthe band edge prevents carriers, electrons or holes, to flow smoothlyand get collected, which then results in poor solar cell performance, asrepresented often by a “roll-over” or “double-diode” effect in thecurrent-voltage (I-V) characteristics.

As a specific example of the problem, single-junction hydrogenatedamorphous silicon (a-Si) based solar cells could be fabricated. In thesea-Si based solar cell (including solar cells based on a-SiGe:H alloys),the absorber layer is sandwiched between two doped layers which generatean electrical field. over the intrinsic layer (i-layer). Either then-type doped layer or the p-type doped layer could serve as the windowlayer, which is on the side the sunlight enters. However, due to thefact the hole mobility is much smaller than the electron mobility ina-Si based materials, the p-layer is often used as the window layer sothat holes, having smaller mobility compared with electrons, will needto travel less distance to get collected. For this reason, theproperties of the p-layer must meet several, often conflicting,requirements. The p-layer must have a wider bandgap so that sunlight canpass through the p-layer without being absorbed before reaching theintrinsic layer (absorber layer in this case) for the photon toelectricity conversion. On the other hand, this p-layer must not have abandgap wider than the i-layer since there would be a mismatch in theband edge at the p-i interface.

In order to make a single-junction solar cell with higher efficiency, itis desirable to reduce the bandgap of the absorber layer, for example byusing alloys having a small amount (about 10-30%) of germanium. Earlierwork by the inventor found that the a-SiGe solar cells with about 10-30%Ge in the i-layer is more stable after prolonged exposure in the sun.The p-layer for such a lower bandgap a-SiGe absorber layer needs to havea smaller bandgap so that the p-layer can form a smooth interface withthe lower-bandgap a-SiGe i-layer while at the same time the p-layerneeds to have a wider bandgap to have minimized absorption.

The problems and difficulties represented here for single-junctiona-SiGe solar cell apply also to a broader range of solar cells that haveat least a doped window layer and a lightly doped or undoped absorberlayer.

Therefore, there is a need to design a novel window layer that overcomesmost, if not all, of the preceding problems.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to a novel photovoltaicsolar cell comprising: at least one absorber layer, and at least onedoped window layer having at least two sub-layers. The firstsub-window-layer is adjacent the absorber layer and forms a desirablejunction with the absorber-layer and the second sub-window-layer isadjacent the first sub-window-layer and has high optical transmission.In certain embodiments, the second sub-window-layer has a transparencygreater than the transparency of the first sub-layer.

In certain aspects, the photovoltaic cell comprises an thin film silicon(tf-Si) alloy based solar cell including at one of amorphous silicon(a-Si:H) based solar cell, nanocrystalline silicon (nc-Si:H) based solarcell, microcrystalline silicon (μc-Si:H) based solar, polycrystallinesilicon (poly-Si:H) based solar cell, or other combinations andmixtures. In certain specific aspects, the photovoltaic cell comprisesan amorphous silicon alloy based solar cell such as, for example, atleast one of a-Si:H, a-Si_((1-x))Ge_(x):H and other combinations andmixtures.

The doped window-layer can comprises a p-type layer or an n-type layerand be formed using vapor phase deposition, such as for example, plasmaenhanced chemical vapor deposition. The desirable deposition conditionsare achieved by varying parameters including at least one of thefollowing: temperature, composition of gas mixtures, rf power, pressure,reactor geometry and dilution with gases such as hydrogen.

The solar cell can further comprise one or more of the following: asubstrate selected from at least one of: glass, metal or plastic; asuitable transparent conductive oxide layer adjacent the secondsub-window-layer; an encapsulation layer overlaying the solar cell toprovide a substantially airtight and watertight protective barrieragainst moisture and contaminants; and/or a buffer semi-conductor layerbetween the absorber-layer and the first sub-window-layer. In certainembodiments, the absorber layer is the i-layer for a-Si solar cells;and, for other solar cells, such as crystalline silicon solar cells, theabsorber layer is a lightly doped layer.

In another aspect, the present invention relates to a method formanufacturing a solar cell comprising the steps of:(i) providing asubstrate; (ii) depositing semiconductor layers that comprise at leastan absorber layer and at least one doped-window-layer, wherein the dopedwindow layer comprises at least two-sub-window-layers deposited underdesirable deposition conditions; and, (iii) depositing a layer oftransparent conducting oxide next to the doped-window-layer but not indirect contact with the absorber-layer. In certain embodiments, thefirst sub-window-layer is adjacent to the absorber layer and isdeposited under conditions which achieve a desirable junction with theabsorber-layer; and in which the second sub-window-layer is adjacent thefirst sub-window-layer but not directly in contact with theabsorber-layer and is deposited under conditions which achieve highoptical transmission.

The doped window layer can be deposited before or after the depositionof the semiconductor absorber layer. In certain embodiments, theabsorber layer contains silicon and germanium and during theabsorber-layer deposition, an optimized ratio of germanium-containinggas and silicon-containing gas provides a Ge content suitable forforming a high efficiency single-junction solar cell. The first andsecond sub-window-layers are deposited by a vapor phase depositionprocess such as, for example, by chemical vapor deposition includingradio frequency plasma enhanced chemical vapor deposition. The plasmaenhanced chemical vapor deposition can be by at least one of thefollowing: cathodic direct current glow discharge, anodic direct currentglow discharge, radio frequency glow discharge, very high frequency(VHF) glow discharge, alternate current glow discharge, or microwaveglow discharge.

In certain aspects, the first and second window-layers amorphoussilicon-containing material are selected from: hydrogenated amorphoussilicon, hydrogenated amorphous silicon carbide, and hydrogenatedamorphous silicon. germanium, as well as the mixtures and combinationsof the above. In certain embodiments, the first and second window-layerssilicon-containing material are selected from: a-Si:H, a-S_(1-x)C_(x):H,a-S_(1-x)Ge_(x):H, nc-Si:H, nc-Si_(1-x)C_(x):H, nc-Si_(1-x)Ge_(x):H,μc-Si:H, μc-Si_(1-x)C_(x):H, μc-Si_(1-x)Ge_(x):H,as well as mixtures andcombinations of the above.

Further, in certain specific embodiments, the present invention isdirected to a photovoltaic solar cell comprising: at least one n-typelayer, at least one i-type layer, and at least two sub-p-layers. Thefirst sub-p-layer, which can also be considered as an interface p-layer,is deposited at a desired first temperature next to the i-type layer anda second sub-p-layer is deposited next to the first sub-p-layer at adesired second temperature which is lower than the first temperature atwhich the first sub-p-layer is deposited. The first sub-p-layer isdeposited next to the i-type layer at a temperature sufficiently high toform a good junction with the i-layer. In certain preferred embodiments,the first sub-p-layer is deposited at about 140° C.

The second sub-p-layer has a transparency greater than the transparencyof the first sub-p-layer. The second sub-p-layer is deposited at atemperature sufficient low to provide acceptable transparency. Incertain embodiments, the second sub-p-layer is deposited at or below atemperature of about 70° C.

The first and second p-layers amorphous silicon-containing material aregenerally selected from the group including hydrogenated amorphoussilicon, hydrogenated amorphous carbon, and hydrogenated amorphoussilicon germanium. In certain embodiments, the i-layer compriseshydrogenated amorphous silicon germanium having a bandgap ranging fromabout 1.4 e-V to about 1.6 e-V and wherein the first and second subp-layers comprise amorphous silicon with a bandgap of 1.6 eV.

Also, in certain embodiments, the first sub-p-layer has a thickness inthe range of about 0.001 micron to about 0.004 micron and the secondsub-p-layer has a thickness in the range of about 0.005 micron to about0.02 micron. It is to be understood that in certain embodiments, thefirst sub-p-layer is thinner than the second sub-p-layer.

The solar cell made using the hybrid sub p-layers has a conversionefficiency of about 10% or greater. Such solar cell can include asuitable substrate such as a glass, metal or plastic, and can furtherinclude a suitable transparent conductive oxide layer adjacent thesecond sub-p-layer. The transparent conductive oxide layer can comprise,for example, indium-tin-oxide (ITO) deposited at a temperaturesufficiently low to avoid damaging the second sub-p-layer. It is furtherto be understood that the solar cell can further comprise anencapsulation layer overlaying the cell to provide a substantially airtight and water tight protective barrier against moisture andcontaminants.

Also, in certain embodiments, the solar cell can further comprise abuffer semi-conductor layer between the n-layer and the i-layer andbetween the i-layer and the first sub-p-layer.

Various materials are especially useful in the present invention. Forexample, the first and second p-layers can comprise an amorphoussilicon-containing material; the i-layer can comprise amorphous silicongermanium; and the n-layer can comprise amorphous silicon.

In another aspect, the present invention relates to a method formanufacturing a solar cell comprising the steps of

-   -   (i) providing a substrate;    -   (ii) depositing a layer of n-type semi-conductor on the        substrate at a temperature sufficiently low to avoid damage or        melting the substrate;    -   (iii) depositing an i-layer on the n-layer at a temperature        sufficiently low to avoid melting or damaging the n-layer;    -   (iv) depositing a first sub-p-layer on the i-layer at a        temperature sufficiently high to form a good junction with the        i-layer; and    -   (v) depositing a second sub-p-layer on the first sub-p-layer at        a temperature lower than the first temperature at which the        first sub-p-layer is deposited.

In certain embodiments, the method can further include depositing alayer of a transparent conductive oxide on the second p-layer. Also acurrent collection layer can be deposited onto the substrate prior todeposition of the n-layer onto the substrate.

In certain embodiments, the deposition sequence of the layers can be asfollowing:

-   -   (i) providing a substrate;    -   (ii) depositing a second sub-p-layer on the substrate at a        temperature relatively low for improved transparency of the        p-layer;    -   (iii) depositing a first sub-p-layer on the second sub-p-layer        at a relatively higher temperature to form a good junction with        the i-layer to be deposited;    -   (vi) depositing an i-layer on the first sub-p-layer at a        temperature sufficiently low to avoid melting or damaging the        p-layer; and    -   (v) depositing a layer of n-type semi-conductor on the substrate        at a temperature sufficiently low to avoid damage or melting the        p and i-layers.

In certain embodiments, during the i-layer deposition step, an optimizedGeH₄ to Si₂H₆ ratio is used to provide a Ge content suitable for forminga high efficiency single-junction solar cell. Still further, in certainembodiments, an optimized level of hydrogen dilution is used to form thei-layer. Appropriate level of hydrogen dilution of process gasesimproves the structural order and photovoltaic quality of amorphoussilicon based materials.

In certain embodiments, the first and second sub-p-layers are depositedby a suitable chemical vapor deposition process such as a plasmaenhanced chemical vapor deposition. The plasma enhanced chemical vapordeposition can be by at least one of the following: cathodic directcurrent glow discharge, anodic direct current glow discharge, radiofrequency glow discharge, very high frequency (VHF) glow discharge,alternate current glow discharge, or microwave glow discharge at apressure ranging from about 0.2 to about 3 with a dilution ratio ofdilutant to feedstock (deposition gas) ranging from TORR about 5:1 toabout 200:1.

In yet another aspect, the present invention relates to a method formanufacturing a solar cell comprising the steps of:

-   -   (i) providing a transparent substrate;    -   (ii) depositing a transparent conducting oxide layer on the        substrate;    -   (iii) depositing a second sub-p-layer on the substrate at a        temperature relatively low for improved transparency of the        second sub p-layer;    -   (iv) depositing a first sub-p-layer on the second sub-p-layer at        a relatively higher temperature to form a good junction with an        i-layer to be deposited thereon;    -   (v) depositing the i-layer on the first sub-p-layer at a        temperature sufficiently low to avoid melting or damaging the        p-layer; and    -   (vi) depositing a layer of n-type semi-conductor on the        substrate at a temperature sufficiently low to avoid damage or        melting the p and i-layers.

Other objects and advantages of the present invention will becomeapparent to those skilled in the art upon a review of the followingdetailed description of the preferred embodiments and the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a J-V graph showing an amorphous silicon with relativelywide bandgap (WBG) top cell having a high performance (V_(oc)=1.023V andFF fill factor 77.5%) obtained using a p-layer deposited at atemperature of 70° C. Such a p-layer forms an ideal junction with a WBGi-layer.

FIG. 1 b is a schematic illustration of amorphous silicon WBG top celldescribed in FIG. 1 a.

FIG. 2 a is a J-V graph showing an amorphous silicon top cell having ap-layer comprising nc-Si, a-Si or a mixed of both phases used for Narrowbandgap a-SiGe solar cells where severe rollover occurs in the J-Vcurve, possibly due to a mismatch at the p-i interface.

FIG. 2 b is a schematic illustration of the type amorphous silicon topcell described in FIG. 2 a.

FIG. 3 a is a J-V graph of a nc-Si/a-Si layer deposited at a highertemperature of 140° C. which forms a good interface with the NBG a-SiGei-layer and leads to an ideal J-V curve. The diode characteristics ofthis material are better than that of the material shown in FIG. 1 a.While the p-layer of the FIG. 3 a material is less transparent than thep-layer of the material shown in FIG. 1 a, such material is yetacceptable for a middle and bottom cell for in a triple stack. Thismaterial, however, would not be acceptable for use in anysingle-junction a-SiGe solar cells

FIG. 3 b is a schematic illustration of the type amorphous silicon topcell described in FIG. 3 a.

FIG. 4 a is a J-V graph showing a hybrid p-layer for a single-junctionmedium bandgap a-SiGe cell which forms a good interface with the a-Sii-layer and is more transparent than the material shown in FIGS. 3 a and3 b.

FIG. 4 b is a schematic illustration of the type amorphous silicon topcell described in FIG. 4 a.

FIG. 5 a is a table for comparative material A, comparative material B,Example 1 of the present invention, and Example 2 of the presentinvention showing the open circuit voltage (V_(oc)), the short circuitcurrent (J_(sc)), the fill factor (FF) and the version efficiency (η).

FIG. 5 b is a schematic illustration of a single junction solar cellhaving the hybrid p-layer of the present invention.

FIG. 5 c is a table showing the deposition conditions for comparativematerial A(GD904), comparative material B (GD907), Example 1 (GD908),and Example 2 (GD919).

FIGS. 6 a, 6 b and 6 c are graphs showing: the dependency of the shortcircuit (J_(SC)) (FIG. 6 a); the open circuit voltage (J_(OC)) (FIG. 6b); and efficiency (EFF) (FIG. 6 c) of n-i-p a-Si-Ge solar cells of GeH₄fractions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a novel p-layer in single-junctionamorphous silicon and silicon germanium alloy photovoltaic elements. Thep-layer of the present invention, when incorporated into solar cells,provides single-junction solar cells with greater than about 12% initialefficiency and greater than about 10% stabilized efficiency. The solarcells with the p-layer of the present invention exhibit efficienciesthat previously could only be achieved using multi-junction solar cellsstructure such as a triple-junction structure. The solar cells of thepresent invention have only ⅓ of the junctions compared with thetriple-junction solar cell and, therefore, can be fabricated withsignificantly lower costs in terms of capital, labor and materials.

The p-layer of the present invention comprises a hybrid p-layer thatcomprises at least two adjacent sub p-layers. A first sub p-layer isdeposited at a desired first temperature and a second sub p-layer isdeposited at another desired second temperature. In this way, the firstsub p-layer, or interface region, forms a good junction with a suitablei-layer, such as an amorphous silicon germanium (a-SiGe) absorber layer.The second sub p-layer is highly transparent, thus allowing moresunlight to reach the semi-conductor absorber layer.

In certain embodiments, the second sub p-layer has a thickness that isat least as thick as, an in certain embodiments, thicker than the firstsub p-layer. Also, in certain embodiments, the ratio of thicknesses offirst sub p-layer to second sub p-layer is about 1:1 to about 1:3. Incertain embodiments, the first sub p-layer is formed to a thickness onthe order of 2 nm and in certain embodiments, preferably less than 4 nmand the second sub p-layer is formed to a thickness on the order of 10nm, preferably less than 20 nm.

It is also to be understood that the thickness of the first and secondsub p-layers can be adjusted to maximize efficiency and equalize thecurrent generated in each layer. Further, it is to be understood thatthe solar cells can have the bandgap of the amorphous silicon layersvaried by adjusting the hydrogen concentration in the amorphous siliconlayers.

The present invention provides a significant improvement in the solarcell conversion efficiency of a low cost amorphous silicon based thinfilm solar cell. With only ⅓ of the junctions needed for atriple-junction solar cell, the devices using the solar cell of thepresent invention can be fabricated at low costs yet with approximatelythe same efficiency, thereby resulting in significant cost savings.

According to one embodiment, the present invention provides highefficiency, single-junction p-i-n a-SiGe photovoltaic material depositedon a suitable substrate to form a solar cell. The solar cell is formedby depositing the p-i-n a-SiGe photovoltaic material onto a substratesuch as stainless steel, with or without textured ZnO/Ag or Zn/Al backreflector by using a suitable deposition process such as radio frequencyplasma enhanced chemical vapor deposition (Rf PECVD). According toanother aspect of the present invention, there is provided a method formaking such solar cells which includes: using an optimized GeH₄ toSi₂H_(s) ratio during the i-layer deposition which leads to a Ge contentespecially suited for high efficiency single-junction a-SiGe cells;using an optimized level of hydrogen dilution for the i-layer; andproviding an improved hybrid p-layer which forms an optimized interfacebetween the p-layer and the a-SiGe i-layer.

FIGS. 1 a 1 b, 2 a, 2 b, 3 a, and 3 b show schematic illustrations anddata for currently used photovoltaic materials and, in particular, theeffect of various types of p-layers on single-junction a-SiGe solarcells. FIGS. 1 a and 1 b relate to a solar cell structure having anamorphous silicon WBG top cell with a high performance (V_(OC)=1.023 vand FF (Fill Factor)=77.5% with a a-Si:H based p-layer deposited at atemperature of 70° C.

FIGS. 2 a and 2 b relate to the same a-Si:H based p-layer used for NGBa-SiGe deposited at 70° C., showing a severe roll-over which occurs inthe J-V curve and which is believed to be due to a band edge mismatch atthe p-i interface.

FIGS. 3 a and 3 b relate to a solar cell where the p-layer is depositedat a higher temperature of about 140° C. Such p-layer forms a goodinterface with the NBG a-SiGe I-layer which, in turn, leads to a desiredJ-V curve. However, the p-layer which is deposited at 140° C. is lesstransparent than the p-layer deposited at 70° C. Such high-temperaturedeposited p-layers are acceptable for middle and bottom cells in atriple junction solar cell. However, such high-temperature depositedp-layer is not acceptable for use in any single-junction solar cells.

Referring now to FIGS. 4 a and 4 b, a schematic illustration of a hybridp-layer for a single-junction medium bandgap solar cells according tothe present invention is shown. The hybrid p-layer forms a goodinterface with the i-layer and has a desirable transparency.

FIG. 4 a is a graph showing the high efficiency a-SiGe cell having aninitial efficiency of about 13% for the a-SiGe single-junction cell anda stabilized efficiency of about 10.4% after 1000 hours of one sunlightsoaking.

FIG. 5 a is a table for comparative material A (gd904), comparativematerial B (gd907), Example 1 of the present invention (gd908), andExample 2 of the present invention (gd919) showing the open circuitvoltage (V_(oc)), the short circuit current (J_(sc)), the fill factor(FF) and the conversion efficiency (η). Both the Example 1 and theExample 2 show favorable fill factor data and conversion efficiency. Itis to be understood that the length of time of the deposition of eachsub p-layer can range from about 0.25 to about 3 minutes and, in certainembodiments, from about 0.5 to about 2 minutes. The deposition rate forthe p-layers shown in FIG. 5 a is about 0.05 nm/sec, or 3 nm/min.

FIG. 5 b is a schematic illustration of a single junction solar cellhaving the hybrid p-layer of the present invention. FIG. 5 c is a tableshowing the deposition conditions for comparative material A(GD904),comparative material B (GD907), Example 1 (GD908), and Example 2(GD919). These devices use heavily doped n-type interface layer at then-ZnO interface, and a heavily doped p-type interface layer at the p-ITOinterface to improve the device FF.

An amorphous silicon-containing thin film semiconductor layer forms asingle junction solar cell. The amorphous silicon semiconductor solarcell comprises a p-i-n amorphous silicon thin film semiconductor with abandgap ranging from about 1.4 eV to 1.75 eV, usually to 1.6 eV. Theamorphous silicon semiconductor material can comprise: hydrogenatedamorphous silicon, hydrogenated amorphous silicon carbon or hydrogenatedamorphous silicon germanium. A second positively doped (p-doped)amorphous silicon-containing sub p-layer is connected to the ITO layerof the front contact. A first positively doped (p-doped) amorphoussilicon-containing p-layer is connected to the second sub p-layer. Thefirst and second sub p-layers can be positively doped with diborane(B₂H₆), BF₃ or other boron-containing compounds. An amorphoussilicon-containing, undoped, active intrinsic i-layer is deposited upon,positioned between and connected to the p-layer and a n-type amorphoussilicon-containing layer. The n-layer is positioned on the i-layer andcan comprise amorphous silicon carbon or amorphous silicon negativelydoped with phosphine (PH₃) or some other phosphorous-containingcompound.

In certain embodiments, the substrate may be made of a single substanceconductive material, or formed with a conductive layer on a supportcomposed of an insulating material or conductive material. Theconductive materials may include, for example, metals such as NiCr,stainless steel, Al, Cr, Mo, Au, Nb, Ta, V, Ti, Pt, Pb, Sn, and alloysthereof. Also, it is to be understood that suitable insulating materialscan include glass, ceramics, papers,. and synthetic resins such aspolyester, polyethylene, polypropylene, polystyrene, polyamide,polycarbonate, cellulose acetate, polyvinyl chloride, polyvinylidenechloride, and the like. The insulating support is formed with aconductive layer on at least one surface thereof, and a semiconductorlayer of the present invention is formed on the surface having theconductive layer formed thereon. For example, when the support is glass,a conductive layer composed of a material such as SnO₂, NiCr, Al, Ag,Cr, Mo, Ir, Nb, Ta, V, Ti, Pt, Pb, In ₂ O₃, ITO (In₂O₃+SnO₂), ZnO, or analloy thereof is formed on the surface of the glass; for a syntheticresin sheet such as polyester film, a conductive layer composed of amaterial such as NiCr, Al, Ag, Pb, Zn, Ni, Au, Cr, Mo, Ir, Nb, Ta, V,Tl, Pt, or an alloy thereof is formed on the surface; and for stainlesssteel, a conductive layer composed of a material such as NiCr, Al, Ag,Cr, Mo, Ir, Nb, Ta, V, Ti, Pt, Pb, In₂O₃ ITO (In₂O₃+SnO₂), ZnO, or analloy thereof is formed on the surface. The thickness of the substratemay be appropriately determined so as to be able to form photovoltaicelements as desired, but when the photovoltaic element is required tohave flexibility, the substrate can be made as thin as possible withinthe range of sufficiently exhibiting the support function.

Doped Layers (p-layer, n-layer)

The base material of the doped layer(s) is composed of non-singlecrystalline silicon type semiconductor. Examples of the amorphous(abbreviated as a-) silicon type semiconductor include a-Si, a-SiGe,a-SiC, a-SiO, a-SiN, a-SiCO, a-SiON, a-SiNC, a-SiGeC, a-SiGeN, a-SiGeO,a-SiCON, and a-SiGeCON.

i-Layer

In the photovoltaic material of the present invention, the i-layer maybe made of amorphous silicon type semiconductor, whether slightly p-typeor slightly n-type. Examples of the amorphous silicon type semiconductorinclude a-Si, a-SiC, a-SiO, a-SiN, a-SiCO, a-SiON, a-SiNC, a-SiCON,a-SiGe, a-SiGeC, a-SiGeO, a-SiGeN, a-SiCON, a-SiGeNC, and a-SiGeCON.

Transparent Electrode

The transparent electrode may be suitably made of a material such asindium oxide (In₂O₃), tin oxide (SnO₂), or ITO (In₂O₃+SnO₂), to whichfluorine may be added. The deposition of the transparent electrode isoptimally performed by a suitable deposition method such as sputteringor vacuum vapor deposition. The vapor deposition sources suitable fordepositing the transparent electrode by vacuum deposition includemetallic tin, metallic indium, and indium-tin alloy.

While the photovoltaic element of the pin structure has been describedabove, the present invention is also applicable to the photovoltaicelements having a laminated pin structure such as a pinpin structure ora pinpinpin structure, or to photovoltaic elements having a laminatednip structure such as an nipnip structure or an nipnipnip structure. Incertain embodiments, the benefit is the greatest for use as the topp-layer in a multijunction solar cell.

The photoelectric conversion element according to the present inventionwill be described below in detail, exemplifying solar cells orphotosensors, to which the present invention is not limitative.

First, a substrate was fabricated. A stainless steel substrate having adesired thickness was cleaned and dried. A light reflection layer of asuitable reflective material such as Ag was formed on the surface of thestainless substrate at room temperature, and then a transparentconductive layer of ZnO was formed thereon using a suitable depositionmethod such that fabrication of the substrate was completed.

The photovoltaic element and method according to the present inventionis described in detail exemplifying a solar cell for photosensors forwhich the present invention is not limitative.

EXAMPLE

The solar cell device structure is SS/back-reflector/a-Si n-layer/n-ibuffer/a-SiGe absorber i-layer/i-p buffer/a-Si based p-layer/ITO. Thea-SiGe absorber layers were deposited using a gas mixture of disilane,germane and hydrogen with a varying germane to disilane ratio and ahydrogen dilution of 5-100. The illumination I-V measurement was takenunder a Xe lamp solar simulator. Quantum efficiency (QE) measurement wasmade in the range of 350-900 nm using a Xe lamp. Light soaking was doneunder AM1.5 light from a metal halide lamp for 1000 hours.

FIGS. 6 a, 6 b and 6 c show the short circuit current (J_(sc)) opencircuit voltage (V_(oc)), and the conversion efficiency (η) of n-i-pa-SiGe solar cells deposited on SS as a function of the [GeH₄]/[Si₂H₆]ratio in the reaction gas, respectively. The solid lines in thesefigures are used only for guiding eyes. It is seen that with increasingGeH₄ fraction, J_(sc) increases, whereas V_(oc) decreases, as a result ηreaches a maximum at an intermediate GeH₄/Si₂H₆ ratio of about 0.3,before decreasing with further increasing GeH₄/Si₂H₆ ratio. Thisintermediate value of GeH₄/Si₂H₆ ratio is close to what is used for thei-layer in the middle cell of standard triple-junction solar cells.

An optimized p-layer ideal for wide bandgap a-Si solar cell is notappropriate for intermediate bandgap a-SiGe solar cells since it leadsto either a low V_(oc) or a poor fill factor. These devices show notonly the decay of FF, but also anomalous rollover behaviors of theilluminated I-V characteristics, caused by the un-optimized p-layer, asshown in FIGS. 2 a and 2 b.

The above detailed description of the present invention is given forexplanatory purposes. It will be apparent to those skilled in the artthat numerous changes and modifications can be made without departingfrom the scope of the invention. Accordingly, the whole of the foregoingdescription is to be construed in an illustrative and not a limitativesense, the scope of the invention being defined solely by the appendedclaims.

1. A method for manufacturing a solar cell comprising the steps of: (i)providing a substrate; (ii) depositing a layer of n-type semi-conductoron the substrate at a temperature that is one of 300° C. andsufficiently low to avoid damage or melting the substrate; (iii)depositing an i-layer on the n-layer at a temperature that is one of300° C. and sufficiently low to avoid melting or damaging the n-layer;(iv) depositing a first sub-p-layer on the i-layer at a firsttemperature sufficiently high to form a good junction with the i-layer;and (v) depositing a second sub-p-layer on the first sub-p-layer at atemperature lower than the first temperature at which the firstsub-p-layer is deposited.
 2. The method of claim 1, further includingdepositing a layer of a transparent conductive oxide on the secondp-layer.
 3. The method of claim 2, wherein a current collection layer isdeposited onto the substrate prior to deposition of the n-layer onto thesubstrate.
 4. The method of claim 1, wherein during the i-layerdeposition an optimized GeH₄ to Si₂H₆ ratio provides a Ge contentsuitable for forming a high efficiency single-junction solar cell. 5.The method of claim 4, wherein an optimized level of hydrogen dilutionis used to form the i-layer.
 6. The method of claim 5, wherein thesubstrate comprises glass or metal including aluminum, bismuth, iron,niobium, titanium or steel.
 7. The method of claim 1, wherein the firstand second sub-p-layers are deposited by a chemical vapor depositionprocess.
 8. The method of claim 7, wherein the chemical vapor depositionprocess comprises plasma enhanced chemical vapor deposition.
 9. Themethod of claim 8, in which the plasma enhanced chemical vapordeposition comprises radio frequency plasma enhanced chemical vapordeposition.
 10. The method of claim 9, wherein the first and secondp-layers amorphous silicon-containing material selected from the groupconsisting of hydrogenated amorphous silicon, hydrogenated amorphouscarbon, and hydrogenated amorphous silicon germanium.
 11. The method ofclaim 10, wherein the i-layer comprises hydrogenated amorphous silicongermanium having a bandgap ranging from about 1.4 e-V to 1.6 e-V andwherein the first and second sub p-layers comprise amorphous siliconwith a bandgap of 1.4 e-V.
 12. The method of claim 11, wherein theplasma enhanced chemical vapor deposition is by at least one of thefollowing: cathodic direct current glow discharge, anodic direct currentglow discharge, radio frequency glow discharge, very high frequency(VHF) glow discharge, alternate current glow discharge, or microwaveglow discharge at a pressure ranging from about 0.5 to about 5 TORR witha dilution ratio of dilutant to feedstock (deposition gas) ranging fromabout 5:1 to about 200:1.